columbia大学有机化学课件25_06_08 html

1、25.6Cyclic Forms of Carbohydrates: Furanose FormsRR C HO+ROHCROO RRProduct is a hemiacetal.Recall from Section 17.8RROHOCCOHOAldehydes and ketones that contain an OH group elsewhere in the molecule can undergo intramolecular hemiacetal formation.The equilibrium favors the cyclic hemiacetal if the ri

2、ng is 5- or 6-membered.Cyclic HemiacetalsCHO1OH4H4CH2OHequilibrium lies far to the rightcyclic hemiacetals that have 5-membered rings are called furanose forms23O132Carbohydrates Form Cyclic HemiacetalsCHO1H HHOHHOH4HOHHOHOH4CH2OHstereochemistry is maintained during cyclic hemiacetal formation23O1H3

3、2D-ErythroseD-Erythrose1234D-Erythroseturn 9041321234D-Erythrosemove O into position by rotating about bond between carbon-3 and carbon-44132D-Erythrose14132432D-Erythroseclose ring by hemiacetal formation between OH at C-4 and carbonyl group1432D-Erythrose14321432CHO1H HHOHHOH4HOHHOHOH4CH2OHstereoc

4、hemistry is variable at anomeric carbon; two diastereomers are formed23O1H32anomeric carbonD-ErythroseH HH HHHHOH44OHHOHOHOHOHa-D-Erythrofuranoseb-D-ErythrofuranoseO1H32O1H32D-Erythrose1 CHOOH OHOHH HH5 CH2OHfuranose ring formation involves OH group at C-4234D-Ribose1 CHO5OH OHOHH HHO4HOHOH5 CH2OHne

5、ed C(3)-C(4) bond rotation to put OH in proper orientation to close 5-membered ringHCH2OH1HHCHO32234D-Ribose55H1OO44HOHOHOHOHHCH2OH1HHCHO32OCH2OHHHCH H32D-Ribose5HOCH2OH1O4OHOHCH2OH group becomes a substituent on ringHHCH H32D-Ribose55HOCH2HOCH2OHOH1 O44HOHOHOHOHb-D-RibofuranoseCH2OH group becomes a

6、 substituent on ringHOH1H32HHCH H32D-Ribose25.7Cyclic Forms of Carbohydrates: Pyranose Forms1 CHO15 CH2OHcyclic hemiacetals that have 6-membered rings are called pyranose forms2345OOH432HCarbohydrates Form Cyclic Hemiacetals1 CHOOHOH OHHH H5 CH2OHpyranose ring formation involves OH group at C-5234D-

7、Ribose1 CHO51 OHOH OHHH HO4HOHOH5 CH2OHpyranose ring formation involves OH group at C-5234HCH2OHHHCHO32D-Ribose51O4HOHOHpyranose ring formation involves OH group at C-5HCH2OHHHCHO32D-RiboseH51O41HHOHOHOHOHb-D-RibopyranoseHCH2OHHHCHO32H5OOH4HHHO32HD-RiboseHH11HHOHOHOHOHa-D-RibopyranoseH5OH4HH HO32OHH

8、5OOH4HHHO32HD-Ribose1 CHO2OH H OHOHH HOHH3456CH2OHpyranose ring formation involves OH group at C-5D-Glucose1 CHO6H2OH H OHOHH HOHH3O4415HHOH6CH2OHpyranose ring formation involves OH group at C-55CH2OHHOHCHOHHO32D-Glucose6HO41HHOHneed C(4)-C(5) bond rotation to put OH in proper orientation to close 6

9、-membered ring5CH2OHHOHCHOHHO32D-Glucose66HHOCHOO4411HHHOHHOHneed C(4)-C(5) bond rotation to put OH in proper orientation to close 6-membered ring5CH2OHHOHCHOHHO322OHH5HOHHCH O32D-Glucose6HOCH26HOCHO141HHHOHHOHb-D-Glucopyranose2OHH5HOHHCH O32H5OOH4HHOHO32HD-Glucose6HOCH26HOCH211HHHOHHOHa-D-Glucopyra

10、noseb-D-GlucopyranoseH5OOH4HHOHO32HH5OH4HH OHO32OHD-Glucose6HOCH21HHOHb-D-Glucopyranosepyranose forms of carbohydrates adopt chair conformationsH5OOH4HHOHO32HD-Glucose6HOCH2H6H1OHHHOHHHb-D-Glucopyranoseall substituents are equatorial in b-D-glucopyranoseH5OOH4HOHHO32HHOCH2 H4O5OHO231HOHD-GlucoseHHHH

11、OHHHHHOHb-D-Glucopyranosea-D-GlucopyranoseOH group at anomeric carbon is axial in a-D-glucopyranoseHOCH2 HOOHO1HOHHOCH2 HOOHO1HOHD-GlucoseCHOOH OHOHH HHCH2OHLess than 1% of the open-chain form of D-ribose is present at equilibrium in aqueous solution.Figure 25.576% of the D-ribose is a mixture of th

12、e a and b- pyranose forms, with the b-form predominatingHHOHHOHOHHOHb-D-Ribopyranose (56%)a-D-Ribopyranose (20%)HHHOOH1HOHHHHOOHHOHFigure 25.5The a and b-furanose forms comprise 24% of the mixture.HOCH2HOCH2OHHHOHOHOHOHOHb-D-Ribofuranose (18%)a-D-Ribofuranose (6%)HOHHHOHHFigure 25.525.8MutarotationM

13、utarotation is a term given to the change in the observed optical rotation of a substance with time.Glucose, for example, can be obtained in either its a or b-pyranose form.The two forms have different physical properties such as melting point and optical rotation.When either form is dissolved in wa

14、ter, itsinitial rotation changes with time.Eventuallyboth solutions have the same rotation.MutarotationHHHHOHHHHHOHb-D-GlucopyranoseInitial:aD +18.7a-D-GlucopyranoseInitial:aD +112.2HOCH2 HOOHO1HOHHOCH2 HOOHO1HOHMutarotation ofD-GlucoseHHHHOHHHHHOHb-D-GlucopyranoseInitial:aD +18.7a-D-GlucopyranoseInitial:aD +112.2aDFinal:+52.5HOCH2 HOOHO1HOHHOCH2 HOOHO1HOHMutarotation ofD-GlucoseHHHHOHHHHHOHb-D-GlucopyranoseInitial:aD +18.7a-D-GlucopyranoseInitial:aD +112.2aDFinal:+52.5HOCH2 HOOHO1HOHHOCH2 HOOHO1HOHMutarotation ofD-GlucoseHHHHOHHHHHOHb-D-Glucopyranosea-D